Sensors are widely used in modern systems to measure or detect physical parameters, such as direction, position, motion, force, acceleration, temperature, and pressure. While a variety of different sensor types exist for measuring these and other parameters, they all suffer from various limitations. For example, inexpensive low field sensors, such as those used in an electronic compass and other similar magnetic sensing applications, may comprise anisotropic magnetoresistance (AMR) based devices. In order to arrive at the required sensitivity and reasonable resistances that mesh well with complementary metal-oxide semiconductors (CMOS), the chip area of such sensors are generally in the order of square millimeters in size. Furthermore, large set-reset pulses from bulky coils of approximately 500 mA are typically required. For mobile applications, such AMR sensor configurations are costly in terms of expense, circuit area, and power consumption.
Other types of sensors, such as magnetic tunnel junction (MTJ) sensors, giant magnetoresistance (GMR) sensors, and the widely used Hall effect sensors have been used to provide smaller profile sensors, but such sensors have their own concerns, such as inadequate sensitivity and the temperature dependence of their magnetic field response. To address these concerns, MTJ, GMR, and AMR sensors have been employed in a Wheatstone bridge structure to increase sensitivity and to reduce the temperature dependent resistance changes. Hall effect sensors have recently become competitive in this type of application through the development of high sensitivity silicon (Si) based sensors coupled with a thick nickel iron (NiFe) magneto-concentrator for amplification of the local magnetic field. These Hall effect devices typically employ the current spinning technique for optimal temperature response, resulting in a larger than desired CMOS footprint for the circuitry associated with the multiplexing between the various tap point functionality. For minimal sensor size, cost and high performance, MTJ sense elements are preferred.
As a result of the manufacturing process variations, low field Wheatstone bridge based magnetic sensors may exhibit a small yet variable residual offset. Temperature shifts, mechanical stress, and the aging of the device may cause small changes in this offset. Furthermore, conventional magnetic sensors have a sensitivity built into the device by factors such as sense layer thickness, shape, and flux concentrator geometry. Therefore, small variations in the manufacturing process may create variations in the sensor parameters and therefore create a need for the magnetic sensors be tested and calibrated for optimal performance.
Wafer probe cards provide the ability to sort good and bad devices at wafer level to avoid the additional expense of continuing the test and assembly process to a later stage before identifying a poorly performing device. By using printed circuit board probe cards, manufacturers obtain electrical data from the IC devices prior to separating, bonding, and packaging each IC device on the wafer. This data permits a manufacturer to monitor the manufacturing process, to respond to processing problems, and to make process adjustments before incurring additional manufacturing costs. Additionally, other ICs in a multichip module that may be paired with a bad device can be preserved for assembly in packages where all devices are known to be good, instead of requiring other good IC's to be disposed of along with the single poorly performing IC.
Testing of three axis magnetic sensors requires a probe card to supply uniform and adjustable magnetic fields in all three directions at the probe tip locations where the magnetic sensors are to be tested. Some known probe card designs use a spiral coil embedded on a printed circuit board to supply magnetic fields; however, these designs have non-uniform fields at the location of the magnetic sensor devices. The probe card geometry is challenging as no parts of the magnetic coil system may penetrate the plane of the wafer, so the well known Helmholtz geometry for high spatial uniformity is not available. A large uniformity is desirable as it allows greater degrees of parallelism in measurement, i.e., the same or very similar field may be applied to multiple devices. Another desirable feature in the design of the coil set for a probe card is lack of a ferromagnetic core as such cores generally have some remanence, resulting in a residual field imposed on the device after a field excursion, and rendering assessment of the sensors hysteresis behavior difficult.
Accordingly, it is desirable to provide an inexpensive low field sensor probe card for testing integrated magnetic sensors. Furthermore, other desirable features and characteristics of the exemplary embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.